The invention relates to OFDM (Orthogonal Frequency Division Multiplexing) methods as are used for example for transmission of digital broadcast radio and television signals via terrestrial radio channels.
OFDM modulation is a digital modulation method; the message signal, which is in digital form, is in this case applied to carrier signals. The data stream to be transmitted is split into r parts, which are sent in parallel on r carriers. This is therefore also referred to as a multicarrier modulation method.
The OFDM method allows high-quality transmission even in difficult transmission conditions, for example in the presence of multipath propagation. In the presence of multipath propagation, reflections are superposed on the main signal, which are offset in time with respect to the directly received signal. Reflections of the transmitted signal occur on obstructions such as buildings or mountains. The individual echoes which arrive successively at the receiver in general have different amplitudes and delay times. After being superposed on the main signal, they result in fluctuations in the complex channel transmission function. If the discrepancies in the delay times are in the same order of magnitude as the duration of the data bits to be transmitted, adjacent bits can interfere with one another. A group of data bits which represent a specific amplitude level is referred to as a symbol. If the delay times of the various echo signals are greater than the value of the symbol duration, this leads to mutual interference of a correspondingly large number of adjacent symbols. This is referred to as intersymbol interference. Since the duration of the transmitted symbols is lengthened, the intersymbol interference can be reduced.
In the OFDM method, this is done by transmitting a plurality of symbols in parallel. If the information to be transmitted is simultaneously modulated, for example, onto 1000 symbols at different carrier frequencies, then one timeslot which originated before the parallelization of all 1000 sequentially transmitted symbols is available for each individual symbol. The frequency band required for transmission of an individual symbol is reduced by the corresponding value. The total bandwidth of all the individual symbols remains approximately constant in comparison with the broadband-modulating single-carrier method. The modulation of the sub-carriers, that is to say of the carriers at the various frequencies, is carried out by means of a digital single-carrier method. Quadrature amplitude modulation (QAM) or quadrature phase shift keying (QPSK) is often used for this purpose.
In the OFDM method, there is a limitation that all the sub-carrier frequencies are orthogonal to one another. This means that all the sub-carrier frequencies are integer multiples of a specific fundamental frequency. The advantage in this case is that the computation rule of inverse Fourier transformation can be used for transformation and the computation rule of Fourier modulation can be used for demodulation. Algorithms for these computation rules are known which can be carried out with relatively few computation operations.
Discrete (inverse) Fourier transformations are generally used for modulation and demodulation purposes since they can be carried out quickly by signal processors. In discrete transformation, signal values relating to discrete points in time are used as input variables instead of a signal which is constant over time.
When carrying out an (inverse) Fourier transformation, sums of a large number of multiplications are calculated. The time period required for the calculation is governed by the number of multiplications to be carried out. Efficient methods for (inverse) Fourier transformation are known, for example the fast Fourier transformation (FFT).
The data rate to be processed for modulation and demodulation of television signals is wide. When using discrete (inverse) Fourier transformation, it depends primarily on the number of multiplications to be carried out. A digital signal processor (DSP) requires a specific time for one multiplication. The total duration for all the multiplications required for transformation of the values of the input variables thus limits the maximum possible data rate. An overview of methods for orthogonal frequency division modulation is given in the article COFDM: An overview by William Y. Zou and Yijan Wu in IEEE Transactions on Broadcasting, Vol. 41, No. 1, March 1995, pages 1 to 8.
The object of the present invention is to specify methods for orthogonal frequency division modulation and demodulation which are carried out by an (inverse) Fourier transformation, and in which fewer multiplications need be carried out than with the known transformation methods.
With regard to modulation, the object is achieved by a method having the features of patent claim 1. With regard to demodulation, the object is achieved by a method having the features of patent claim 3.
The invention has the advantage that, if the processing speed of a processor remains constant, a higher data rate can be achieved for modulation and demodulation.
A further advantage is that existing OFDM systems which operate using fast Fourier transformation can be upgraded without changing the signal flow in the system in order to carry out the method according to the invention.
Advantageous refinements and developments are characterized in dependent claims.